Generally, MOS transistors are categorized as field effect transistors (FETs). In such a MOS transistor, source and drain regions are formed in a semiconductor substrate. A gate oxide layer and a gate are then formed on the same semiconductor substrate. In such a MOS transistor, metal lines are connected to the electrodes of the source, the drain, and the gate, to receive electrical signals for operating the device.
In a manufacturing process of such a MOS transistor, a gate oxide layer and a polysilicon layer of a predetermined width are deposited on the surface of an active region of a silicon wafer. These layers are then patterned to form a gate electrode. Using the gate electrode as a doping mask, a P-type or N-type dopant is then implanted at a low concentration in a device region of the silicon wafer, such that a lightly doped drain (LDD) region is formed in the device region of the silicon wafer. After forming side walls on both lateral sides of the gate electrode, a dopant having the same conductivity type as the aforementioned LDD region is implanted in the device region of the silicon wafer using the side walls and the polysilicon as a doping mask.
As semiconductor devices have become more integrated, the line widths of the corresponding circuits have become narrower. Accordingly, the size of the gate has become smaller, and a so-called nanogate has been achieved.
However, a gate of a very small size, such as a nanogate, is very difficult to realize using a conventional method of manufacturing a semiconductor device due to the limited capability of a photolithography process for forming the gate.
To clarify multiple layers and regions, the thickness of the layers are enlarged in the drawings. Wherever possible, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. As used in this patent, stating that any part (e.g., a layer, film, area, or plate) is in any way positioned on (e.g., positioned on, located on, disposed on, or formed on, etc.) another part, means that the referenced part is either in contact with the other part, or that the referenced part is above the other part with one or more intermediate part(s) located therebetween. Stating that any part is in contact with another part means that there is no intermediate part between the two parts.